ABSTRACT The overall goal of this project is to gain further understanding of the mucosal immune system and the defects that contribute to the pathogenesis of human inflammatory bowel disease (IBD). The generation of a number of murine models of IBD has facilitated investigation into the basic mechanisms underlying IBD pathogenesis. Despite the fact that many murine models of IBD result from a defect in a single protein known to affect leukocyte function, the specific auto-reactive and/or regulatory cell population responsible for disease pathogenesis in these models remains unknown. We recently generated a mouse model of IBD that results from the targeted disruption of the Wiskott-Aldrich syndrome protein (WASP). WASP is expressed solely in hematopoietic cells and is a signaling molecule that regulates cell surface receptor signals to the cytoskeleton. Abnormalities in this protein lead to the rare X-linked primary immunodeficiency that carries its name. Autoimmunity is commonly associated with this immunodeficiency, and up to 10 percent of patients develop an IBD-like illness. Our lab has recently shown that 100% of WASP KO (WKO) mice also develop colitis. Genetic and adoptive transfer studies have shown that lymphocytes are essential and CD4+ lymphocytes are sufficient for colitis development. In contrast with most murine models of IBD that have a Th1 bias, we have demonstrated a Th2 cytokine skewing (with elevated IL-4 and IL-13) in lamina propria (LP) lymphocytes and in tissues. Indeed, IL-4 is required, at least in part, for colitis development. Perhaps, most interestingly, we have recently shown that there is marked reduction in the development and function of regulatory T cells in WKO animals. Our overall hypothesis is that the colitis in WKO mice results from both intrinisic nTreg dysfunction as well as intrinsic T cell signaling abnormalities within the CD4+ T cell population that leads to Th2 cytokine skewing. In this proposal we seek to determine the cellular and molecular basis for colitis development and autoimmunity in WKO mice. Our first aim is to test the hypothesis that adoptive transfer of WT nTregs or expansion of residual mutant WKO nTregs will cure chronic colitis of WKO mice. Our second aim is to dissect the mechanism(s) that are responsible for the aberrant nTreg development and dysfunction in WKO mice. Our third aim is to evaluate the molecular basis for cytokine skewing in WKO CD4+ effector T cells and its role in disease development. The overall goal of this proposal is to further our understanding of the mechanism of colitis in WKO mice that is uniquely associated with both profound regulatory T cell defects and Th2 cytokine skewing and to take advantage of a murine model of colitis that uniquely has a human correlate. Our driving premise is that such knowledge will not only aid in the understanding and treatment of the debilitating autoimmunity (and IBD-like illness) that characterizes the human immunodeficiency but will also aid in our understanding of mucosal immune regulation in general and the molecular underpinnings of inflammatory bowel disease.